Dithianes dithioacetal

The dithiane-derived anion can be generated by the action of Bu°U in THF at -78 C or with complex bases NaNH2-RONa at room temperature. Lithiated dithiane can also be prepared in situ by sonica-tion of n-butyl chloride with lithium in the presence of dithiane. Dithioacetals or ketals are resistant to acidic or basic hydrolysis. Regeneration of the carbonyl group from the dithioketal sometimes presents difficulties but can be carried out by hydrolysis in polar solvents (acetone, alcohols, acetonitrile) in the presence of metallic ions such as Hg , Cu, Ag, 71 177 qj j m ise Alternatively, alkylative hy lysis... 

The pivotal step in this sequence is an electrophilic substitution on indole. Although the use of l,3-dithian-2-yl carbanions is well documented, it has been shown only recently that 1,3-dithian-2-yl carbenium ions can be used in a Priedel-Crafts type reaction. This was accomplished initially using 2-methoxy-l,3-dithiane [1,3-Dithiane, 2-methoxy-] or 2-metlioxy-l,3-dithiolane [1,3-Dithiolane, 2-methoxy-] and titanium tetrachloride [Titanate(l —), tetrachloro-] as the Lewis acid catalyst.9 2-Substituted lysergic acid derivatives and 3-substituted indoles have been prepared under these conditions, but the method is limited in scope by the difficulties of preparing substituted 2-methoxy-1,3-dithianes. l,3-Dithian-2-yl carbenium ions have also been prepared by protonation of ketene dithioacetals with trifluoroacetic acid,10 but this reaction cannot be used to introduce 1,3-dithiane moieties into indole. 

The use of ethyl ethylthiomethyl sulphoxide in this reaction leads to the desired addition products in much better yields (95-97%). These products were then converted into ketene dithioacetal monoxide derivatives 430 by a sequence of reactions (equation 258)505. Reaction of 2-lithio-l,3-dithiane-l-oxide with benzophenone affords a mixture of the diastereoisomeric tertiary alcohols 431 in a ratio which is temperature dependent (cis trans changes from 3 1 at — 78 °C to 1 1 at room temperature)268. 

The C-H bond-dissociation energies Z>c-h of the acetals 130 and 131, monothioacetal 132, and the dithioacetal 133 (Figure 7) were calculated from experimental reaction kinetic data <2005MI379>. The C-H dissociation energies of the 1,3-dithiane derivative 133 proved to be lowest, followed by the 1,3-oxathiane derivatives 132,... 

From the synthetic point of view, the most important a-sulfinyl carbanions appear to be those derived from dithioacetal S-oxides which are a synthon for acyl anions65. However, the yields of the alkylation reaction were found to be low. In spite of the fact that dithiane S-oxides have been intensively studied66 63, their synthetic applications are limited,... 

The stoichiometric equivalents of halofluorides have been recently applied to transform alkylene dithioacetals into gcm-difluorides.70-71 Dithioacetals such as 1,3-dithiolanes and 1,3-dithianes arc readily obtained from the corresponding carbonyl compounds by the reaction with ethane-1,2-dithiol or propane-1,3-dithiol in the presence of the complexes boron trifluoride-bis(acetic acid) or boron trifluoride-diethyl ether. Using a two-step procedure, a range of aldehydes and ketones can be converted into gem-difluorides under mild conditions. 

The orf/to-formylation of 2-aminopyridines can be effected via the rearrangement of the azasulfonium salt prepared from a 2-aminopyridine, 1,3-dithiane, f-butyl hypochlorite and sodium methoxide (74CC685). The crude sulfilimine (815) was refluxed in f-butanol containing potassium f-butoxide to yield the dithioacetal (816). Hydrolysis of (816) with mercury(II) oxide/boron trifluoride etherate gave the aldehyde (817 Scheme 191). This method should be applicable to the formylation of other heterocyclic amines. 

The dithiane derivative 60 (Scheme 14) is such a compound, it being made from 2,3 5,6-di-0-isopropylidene-D-mannose by treatment with 2-lithio-l,3-dithiane to give a heptose dithioacetal that was refunctionalized at C-2-C-3 by way of the C-l anion and then converted to the 6,7-epoxide following selective acid-catalyzed cleavage of the 6,7-acetal ring. Treated with n-butyllithium it gives, in 70% yield, the cyclized 61, which is efficiently convertible into validatol 62, a component of validamycin A, by desulfurization with Raney nickel and de-O-protection by use of boron tribromide in dichloromethane [31]. 

Many dithioacetals, useful for synthetic purposes, are now commercially available, including bis(phenylthio)methane (1), dithiane itself and a number of 2-substituted dithianes (2), but also more exotic ones such as (3). (In contrast to the stench of (1), compound (3) is non-odoriferous )... 

Dithioacetals (see also dithianes and dithiolanes) alkylation of 98 as acyl anion equivalents 75 carbanions of 87,97-102 cleavage of 14-18,98,102 desulfurization of 78 metal-catalysed coupling 127 reaction with Grignard reagents 127 reductive lithiation of 89 synthesis of 12-19,97-102 Dithioacids synthesis of 40... 

The syntheses of dithioacetals are generally straightforward [43]. Standard methods may be unselective for multifunctional molecules. Therefore, new procedures have been developed. It has thus been reported that 1,3-dithianes are readily synthesized by reaction of aldehydes, ketones or acetals with 2-stanna-l,3-dithianes under catalysis of organotin triflates [45]. These odourless reagents are prepared from dialkyldichlorotin and 1,3-propanedithiol. 

The dithioacetal (0.01 mol) was stirred for a few hours at room temperature with clayfen (4) (10.4 g, 11 mmol of ferric nitrate) or with claycop (12.1 g, 20 mmol of copper nitrate) in toluene, n-pentane or, preferably, dichloromethane (120ml). Evolution of nitrogen oxides occurred rapidly. Stirring was maintained until gas evolution ceased. The clay was then filtered off and washed twice with portions (50 ml) of the solvent. The resulting pale-yellow or slightly green solution was filtered through a small quantity of neutral aluminium oxide and the solvent was evaporated under vacuum. In the case of dithiane and dithiolane derivatives, this afforded the pure carbonyl compound in excellent yield. 

The oxidation products of dithiols are insoluble and removed with the clay residues by the filtration step, and the method appears particularly suitable for dithioacetals derived from dithiols as dithianes for instance. For other dithioacetals a further purification step was necessary. 

C-C BOND-FORMING REACTIONS 4.2.1 Use of sulfides, dithioacetals and orthotrithioesters 4.2.1.1 1,3-Dithianes as acyl anion equivalents... 

Keywords aldehyde, thiol, lithium bromide, dithioacetalization, 1,3-dithiane, 1,3-dithiolane... 

The most general approach to the synthesis of acyl silanes is based on hydrolysis of 2-silyl-l,3-dithioacetals, first investigated for 1,3-dithiane derivatives by Brook56 and Corey57 in the late 1960s (Scheme 3). Oxathioacetals and protected hemithioacetals have also been used, and some formyl silanes have been isolated by hydrolysis of dioxa-acetals (vide supra). 

In a recent approach to the preparation of dihydro-3(2H)-furanones by chemists from Thailand it was shown that consecutive, one-pot conventional treatment of 1,3-dithiane in THF with equimolar n-butyllithium, an epoxide, n-butyllithium, an aldehyde or ketone and, finally, methanesulfonyl chloride gave a spirocyclic dithioacetal in good (ca. 70%) yield. Hydrolysis with HgO/HgCl2 gave the corresponding dihydrofuranone. 

The reaction of aldehydes or ketones with ethane-1,2-dithiol or propane-1,3-dithiol to form 1,3-dithiolanes or 1,3-dithianes is an important reaction, as these compounds under suitable conditions are acyl anion equivalents (see Section 5.9, p. 626). These cyclic dithioacetals have been less used as protective groups, though when required are formed in high yield in the presence of boron trifluoride-etherate.138... 

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